Process for exchange of charge and apparatus for carrying out the process

ABSTRACT

Positive ions of a gas are charged with electrons from a metallic vapor to provide negative ions of the gas by passing the positive ions through an isothermal enclosure into contact with a metallic vapor providing electrons. The metal providing the vapor is circulated in closed cycle in the enclosure in vapor form from a hot central portion to cooler end portions thereof and in liquid phase from the end portions of the enclosure toward the central portion of the enclosure.

States Patent 1191 Bacal et al.

1111 r 3,825,790 1451 July '23, 1974 [54] PROCEss FOR EXCHANGE OF CHARGE AND APPARATUS FOR CARRYING OUT THE PROCESS [75] Inventors: Martha Bacal, Paris, France; Walter Reichelt, Los Alamos, N. Mex.

[73] Assignee: Commissariat A LEnergie Atomique, Paris, France [22] Filed: Oct. 16, 1972 21 App]. N0.2 297,660

[30] Foreign Application Priority Data Oct. 19, 1971 France 1. 37448 [52 us.C1.... 1. 31[3; ,3131231 51 nt. C1. 1101, 27 00 [58] Field of Search [56] References Cited UNITED STATES PATENTS Middleton eta]. 3l3/ 63 Primary Examinerllerrnan Karl Saalbach Assistant ExaminerDarwin R. Hostetter Attorney, Agent, or Firm--Cameron, Kerkam, Sutton,

' Stowell & Stowell 57 ABSTRACT Positive ions of a gas are charged with electrons from a metallic vapor to provide negative ions of the gas by passing the positive ions through an isothermal enclosure into contact with a metallic vapor providing electrons. The metal providing the vapor is circulated in closedvcycle in the enclosure in vapor form from a hot central portion to cooler end portions thereof and in liquid phase from the end portions of theenclosure toward the central portion of the enclosure.

7 6 Claims, 2 Drawing Figures PATENTEU JUL23I974 FIG. 2

. 1 PROCESS FOR EXCHANGE OF CHARGE AND APPARATUS FOR CARRYING'OUT THE PROCESS BACKGROUND OF THE INVENTION The present invention relates to an exchange process for electric charge and apparatus for carrying out the process T 1 More precisely, the process fixes on positive-ions of a gas, for example helium, electrons given off by ametallic vapor, for example, lithium, to obtain negative ions of the gas, this process prolonging the functional life of the apparatus carrying out the process.

Such a processis particularly useful in the case where negative ions are to be introduced into an accelerator of particles, for example, a Van de Graff Tandem under form-of positive ions from a source.

In general, this exchange ofv charge is produced in a substantially isothermal enclosure. The enclosure contains the vapor of lithium and the flux of positive ions passesthrough the enclosure.

One of the principal problems resides in the fact that because of the interaction of the helium ions with the atoins of lithium, and because of the very low pressure obtained in the tube. This sleeve with the metallic mesh in theaccelerator there is a directed flux of lithium SUMMARY OF THE INVENTION The present process for creation of negative ions of a gas from positive ions of .the gas passes a current of positive ions from a source of positive ions through an enclosure of elongated form containing a metallic vapor susceptible of giving off electrons in which totrap thevapor in the enclosure there is created in the central part of the enclosure a hot zone and at each of the extremities of the enclosure a cold zone, thesubstance providing the vapor to be trapped in the enclosure being circulated in a closed cycle in vapor phase from the hot zone towards the cold z'ones a nd in liquid phase from the cold zones towardsthe hot zone, this last part of the cycle being actuated by capillary forces created on the walls of the enclosure.

The length of life of the apparatus is limited by the capacity of the reservoir of lithium and the losses of lithium at the entry and exit orifices, as well as by the speed of deposit of lithium at the orifices and formerly did not exceed 100 hours. M

In accordance with the present invention, the lithium in vapor phase and in liquid phase circulates in the enclosure for exchange of charge in a closed cycle which provides a longer life for the apparatus.

The central region of the enclosure or exchange duct for the charge is heated to a temperature at which the desired pressure of the lithium vapor is obtained. An

isothermal zone is created by heating a portion of the tremities of the duct is in liquid phase while its vapor pressure at the extremities of the duct is quite low so that the losses of lithium at the input and output orifices is neglibile.

The liquid lithium, condensed at the extremities of the duct for the exchange of charge, diffuses from the cold extremities of the exchange duct toward the hot central zone by' capillary action, the interior wall of the exchange. duct being coveredwith a metallic mesh or wick in accordance with the present invention.

The present invention equally has for an object apparatus for carrying out this process. The apparatus is in cluded by a tube covered on its interior surface by a metallic mesh or wick saturated with a metal which is vaporized within the tube, the tube forming the duct for exchange of charge. A hollow sleeve surrounds the tube over a part of its length and closed at its extremities, the sleeve being covered on its interior surface by a metallic mesh or wick containing a metal having a boiling temperature higher than the temperature to be example, cesium, acting as a heating agentThe exterior wall of the sleeve acts as an evaporator and is maintained at an appropriate temperature by an exterior application of heat. The interior wall of the sleeve, that is the wall of the tube containing lithium, acts as a condensor. Y

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING The present invention will be furtherdescribed by the following description of a preferred embodiment of the invention which in no way limits the scope thereof. In the accompanying drawing, FIG. 1 is a. crosssectional view of an isothermal enclosure containinga vapor and heated by an annular caloduct; and

FIG. 2 is a graphic representative of the distribution of temperatures within the enclosure of FIG. 1.

' SCRIPTION or THE PREFERRED EMBODIMENT Referring now to FIG. 1, the enclosure includes a tube 2 preferably of stainless steel which acts as a duct for exchange of charge. It connects a source 5 of positive helium ions, of known type, to the enclosure E for utilization of negative helium ions, this enclosure preferably being a Van de Graaff cell. Tube 2 is covered on its interior surface with a mesh or wick 4 preferably of stainless steel. At each extremity of tube 2 and outside of the tube are located two heaters 6 and 8, these heaters beingiany suitable means providing an adequate adjustable temperature. Over the central part of tube 2 is secured a sleeve 10 closed at its ends. The interior wall 12 of sleeve IOcoincides with the exterior surface of tube 2. Sleeve 10 is preferably of stainless steel. Sleeve 10 is covered on its interior surface by a mesh 14 preferablyof stainless steel. When in operation, sleeve 10 is evacuated and filled with a cesium vapor 16. Cesium, having a boiling temperature of 670C. can be replaced by other metals to obtain other temperatures in tube 2. For example, potassium can be used having a boiling temperature of 880C.

The exterior wall 18 of sleeve is heated as shown in FIG. 1 by the arrows. The source of this heat can be radiation, induction, or by any other appropriate means. Exterior wall 18 acts as an evaporator for the cesium contained in sleeve 10 while the interior wall 12 of the sleeve acts as a condensor. A uniform temperature is thus provided in all portions of tube 2 surrounded by sleeve 10. The range of temperatures obtained in the portion of tube 2 surrounded by sleeve 10 depends on the source of heat and on the metal utilized in sleeve 10. Outside of this zone there is a temperature gradient as seen in FIG. 2, the abcissa representing distance and the ordinant representing the temperatures.

In the region of tube 2 surrounded by sleeve 10 the temperature is uniform and the pressure of the lithium vapor is identical at every point in this region of tube 2. This is an important characteristic of the apparatus and may be critical in the case where the thickness of the gaseous metallic target should be known.

The lithium atoms tend to leave the hot zone and .locate on the walls of tube 2 close to its extremities. By capillary action created by mesh or wick 4 which covers the interior of tube 2, this deposit of lithium is forced toward the hot zone of tube 2. Stoppage of tube 2 is thus prevented. It is essential that the temperature of operation of heaters 6 and 8 be greater than the temperature of fusion of the metal to be vaporized even though quite close to it, so that the capillary forces can occur.

The functioning of the embodiment of FIG. 1 is apparent from the above description. Lithium is charged into tube 2. Sleeve 10 is partially filled with cesium. A source of heat is applied to sleeve 10 as indicated by the arrows. Cesium evaporates on wall 18 of the sleeve at the contact of the source of heat. Cesium vapor condenses on the interior wall 12 of the sleeve and supplies heat to the central zone of tube 2. Condensed cesium is directed toward wall 18 of sleeve 10 because of mesh 14 through capillary action. The lithium under the action of this transfer of heat vaporizes in the central zone of tube 2. The apparatus'is then ready for use since it is in communication with the source of ions S and enclosure E of the accelerator. The flow of positive ions passing through the lithium vapor captures electrons and the ions become negative. The lithium vapor follows the described cycle between the hot central zone and the cold zones at heaters 6 and 8.

When the lithium vapor is used for exchange of charge, the temperature of operation of the central zone of tube 2 is 500C. In this case, heaters 6 and 8 operate at 220C.

The present invention is not limited to the use of lithium as the gaseous metallic target for exchange of charge of helium ions. Lithium can be replaced in tube 2 by another metal such as cesium, potassium and sodium. With these other metals, the operational temperature would be selected by variation of heat flux applied to the evaporator of the annular caloduct that is sleeve 10, and by suitable choice of the heat transfer agent of the caloduct such as water, cesium, potassium and sodium.

Further, the helium positive ions can be replaced by positive ions or fast atoms of any other element for which negative ions or fast atoms are desired by exchange of charge.

The present invention is not limited to the preferred embodiment described above and shown in the drawing s. Variations in this concept come within the present invention. In particular, isothermal enclosures can be used other than the exchange duct for the charge of a Van de Graaff Tandem. The invention can, for example, be used in the measurements of atomic and ionic collisions, in nuclear fusion research and in reactors with nuclear fusion.

We claim:

1. Process for creation of negative ions in a gas from positive ions in the gas comprising the steps of passing a current of positive ions from a source of positive ions through an elongated enclosure containing a metal providing a metallic vapor capable of giving off electrons, trapping the vapor in the enclosure by creating in the 'central part of the enclosure a hot zone and by creating at each extremity of the enclosure a cold zone, causing the metal whose vapor is trapped in the enclosure to circulate in a closed cycle in vapor phase in the hot zone toward the cold zones and in liquid phase in the cold zones toward the hot zone, and causing circulation from the cold zones toward the hot zone by capillary forces created on the walls of the enclosure.

2. Process as described in claim 1, the cold zones being maintained at a temperature slightly greater than the temperature of fusion of the metal providing the vapor in the enclosure.

3. Apparatus creating negative ions in gas from positive ions in the gas comprising a tube, a metallic wick covering the interior surface of said tube saturated with a metal to be vaporized and maintained within said tube, a source of positive ions, an enclosure for utilization of negative ions, said tube connecting said source and said enclosure, a caloduct comprising a hollow sleeve surrounding and extending over a part of the length of said tube and closed at its ends, a metallic wick covering the interior surfaces of said sleeve and containing a substance having a boiling temperature greater than the operating temperature of said tube and having at the operating temperature a vapor pressure sufficientfor the correct functioning of the caloduct and means at each extremity of said tube for maintaining the extremities of said tube at a temperature slightly higher than the temperature of fusion of the metal to be maintained in said tube.

4. Apparatus as described in claim 3, said means at each extremity of said tube being a heater on the exterior wall of said tube.

5.. Apparatus as described in claim 3, the substance within said sleeve being selected from the group consisting of water,'cesium, sodium, potassium and lithium.

6. Apparatus as described in claim 3, the metal in said tube being lithium, said tube, said sleeve and said metallic wicks being stainless steel. 

2. Process as described in claim 1, the cold zones being maintained at a temperature slightly greater than the temperature of fusion of the metal providing the vapor in the enclosure.
 3. Apparatus creating negative ions in gas from positive ions in the gas comprising a tube, a metallic wick covering the interior surface of said tube saturated with a metal to be vaporized and maintained within said tube, a source of positive ions, an enclosure for utilization of negative ions, said tube connecting said source and said enclosure, a caloduct comprising a hollow sleeve surrounding and extending over a part of the length of said tube and closed at its ends, a metallic wick covering the interior surfaces of said sleeve and containing a substance having a boiling temperature greater than the operating temperature of said tube and having at the operating temperature a vapor pressure sufficient for the correct functioning of the caloduct and means at each extremity of said tube for maintaining the extremities of said tube at a temperature slightly higher than the temperature of fusion of the metal to be maintained in said tube.
 4. Apparatus as described in claim 3, said means at each extremity of said tube being a heater on the exterior wall of said tube.
 5. Apparatus as described in claim 3, the substance within said sleeve being selected from the group consisting of water, cesium, sodium, potassium and lithium.
 6. Apparatus as described in claim 3, the metal in said tube being lithium, said tube, said sleeve and said metallic wicks being stainless steel. 